The specific aims of this proposal are to continue the development of a bank of frozen embryos obtained from valuable genetic stocks maintained at the Jackson Laboratory; to determine the importance of environmental or seasonal effects on the ability to superovulate mice representing different genotypes; to determine the importance of the genetic relationship between embryo and pseudpregnant foster mother upon embryo transfer; and to determine the importance of seasonal environmental variations on the rate of live-born recovery after embryo transfer. Eight-cell embryos will be collected from superovulated female donors, frozen and stored in liquid nitrogen. At least 1,000 embryos from each of over 400 different stocks will be stored in two separate liquid nitrogen refrigerators. Breeding pairs of mice recovered from frozen storage will be provided at cost to qualified investigators when such mice are not elsewhere available. The long-term objective of this project is to establish a bank of frozen embryos so as to provide assurance of the preservation of mutants and strains of mice that are valuable to many fields of research including genetics, cancer, immunology, development and reproduction, aging, blood research, behavior, and neurobiology. The principal purpose of this frozen embryo bank is to preserve irreplaceable murine germplasm, but other beneficial purposes are to reduce the necessary number of different stocks or size of colonies maintained by conventional breeding procedures and to retard genetic drift.